WASHINGTON (CNN) -- FBI Director Robert Mueller told Congress Thursday that the confrontation between then-White House Counsel Alberto Gonzales and then-Attorney General John Ashcroft in Ashcroft's hospital room in 2004 concerned a controversial surveillance program -- an apparent contradiction of Senate testimony given Tuesday by Gonzales.

Mueller said he spoke with Ashcroft soon after Gonzales left the hospital and was told the meeting dealt with "an NSA [National Security Agency] program that has been much discussed, yes."

Mueller made the comment as he testified before the House Judiciary Committee.

In testimony before the Senate Judiciary Committee on Tuesday, Gonzales, now attorney general, said he had visited the ailing Ashcroft in the hospital to discuss "other intelligence activities," not the surveillance program. Watch Mueller's testimony »

Mueller also testified Thursday that he had serious reservations about the program, which allowed surveillance without warrants, at the time of the dramatic internal administration showdown and threats of top-level resignations.

Mueller did not confirm he had threatened to resign, but he twice said he supported the testimony of former Deputy Attorney General James Comey, who testified that Gonzales and former White House Chief of Staff Andrew Card tried to pressure Ashcroft to reauthorize a surveillance program against terror suspects.

Mueller for the first time publicly confirmed he dispatched -- as Comey testified -- an FBI security detail to Ashcroft's hospital room to ensure that Comey was not removed from the room when Gonzales was there.

Earlier Thursday, four Democratic senators called for an independent special counsel to investigate whether Gonzales perjured himself during Capitol Hill testimony.

It's the latest salvo in a dispute regarding President Bush's domestic surveillance program and the Justice Department's firings of U.S. attorneys last year.

Separately, Sen. Pat Leahy, D-Vermont, chairman of the Judiciary Committee, announced he intends to subpoena President Bush's top political adviser, Karl Rove, to testify about the dismissals.

The special prosecutor would be appointed by the solicitor general at the Department of Justice, which Gonzales heads. Both Gonzales and Deputy Attorney General Paul McNulty have recused themselves, putting the matter in the hands of Solicitor General Paul Clement.

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"The attorney general took an oath to tell the truth, the whole truth and nothing but the truth. Instead he tells the half truth, the partial truth and everything but the truth -- and he does it not once, not twice, but over and over and over again," said Sen. Chuck Schumer, D-New York, at a news conference.

"We do not make this request lightly," the senators wrote in a letter to Clement. "We believe a special counsel is needed because it has become apparent that the attorney general has provided -- at a minimum -- half-truths and misleading statements."

These relate to his involvement in the "removal and replacement of U.S. attorneys, about his role in trying to circumvent Acting Attorney General Comey and about the administration's position on the NSA wiretapping program."

The Bush administration has insisted that the firing of the attorneys was handled properly, but critics have charged the attorneys were forced out for political motives and, in one case, to allow a protege of Rove to take one of the posts.

In addition to Schumer, Sens. Dianne Feinstein of California, Sheldon Whitehouse of Rhode Island and Russ Feingold of Wisconsin signed the letter.

"This has been a long and sorry episode for the Department of Justice," Whitehouse said.

In response to the developments, White House Deputy Press Secretary Tony Fratto said, "Every day congressional Democrats prove that they're more interested in headlines than doing the business Americans want them to do. And Americans are now taking notice that this Congress, under Democratic leadership, is failing to tackle important issues."

Sen. Arlen Specter of Pennsylvania, the ranking Republican on the Senate Judiciary Committee and a harsh Gonzales critic, said he doesn't support Schumer's request for a special prosecutor.

Specter accused the senators who signed the letter of moving too quickly on allegations that are "very, very serious," without consulting the rest of the panel. He noted that Leahy's name was missing on the letter.

"I think the fact that he [Leahy] has not signed it is highly significant," Specter said. "I don't think you rush off and ask for appointment of a special counsel to run that kind of an investigation."

"I think there's a little bit of Don Quixote here. People are riding off in all different directions at once. Everybody is trying to top everybody else," Specter said. The president, he added, "feels very strongly that there's been nothing improper here."

The four senators asked that the inquiry focus on three issues:

Gonzales testified February 6, 2006, that "there has not been any serious disagreement about the [no-warrant surveillance program]." However, he testified Tuesday that the purpose of a March 10, 2004, meeting between White House officials and congressional leaders -- referred to as the "Gang of Eight" -- was to inform lawmakers that Comey would not approve "a very important intelligence activity." That was when Comey was filling in for the hospitalized Ashcroft. Another official testified that the 2004 meeting was about the TSA. The senators called the contradiction in Gonzales' statements "deeply troubling."

When the issue came up again, John Negroponte, then director of national intelligence, described the "Gang of Eight" meeting as being about the surveillance program.

On April 19, 2007, when discussing his role in the firings of the attorneys, Gonzales testified, "I haven't talked to witnesses because of the fact that I haven't wanted to interfere with this investigation." However, Monica Goodling, former counsel to Gonzales, testified that while she worked for him, she had an "uncomfortable" conversation with the attorney general "where he outlined his recollection of what happened" and asked her for her reaction.